M E M O RY S ECTIONS

M E M O RY S ECTIONS

The total memory system in a PLC is actually composed of two different memories (see Figure 5-1):

• the executive memory • the application memory

Executive Memory Area

Application Memory Area

Figure 5-1. Simplified block diagram of the total PLC memory system.

The executive memory is a collection of permanently stored programs that are considered part of the PLC itself. These supervisory programs direct all system activities, such as execution of the control program and communica- tion with peripheral devices. The executive section is the part of the PLC’s

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memory where the system’s available instruction software is stored (i.e., relay instructions, block transfer functions, math instructions, etc.). This area of memory is not accessible to the user.

The application memory provides a storage area for the user-programmed instructions that form the application program. The application memory area is composed of several areas, each having a specific function and usage. Section 5-4 covers the executive and application memory areas in detail.

5 -2 M E M O RY T YPES

The storage and retrieval requirements for the executive and application memory sections are not the same; therefore, they are not always stored in the same type of memory. For example, the executive requires a memory that permanently stores its contents and cannot be erased or altered either by loss of electrical power or by the user. This type of memory is often unsuitable for the application program.

Memory can be separated into two categories: volatile and nonvolatile. Volatile memory loses its programmed contents if all operating power is lost or removed, whether it is normal power or some form of backup power. Volatile memory is easily altered and quite suitable for most applications when supported by battery backup and possibly a disk copy of the program. Nonvolatile memory retains its programmed contents, even during a com- plete loss of operating power, without requiring a backup source. Nonvolatile memory generally is unalterable, yet there are special nonvolatile memory types that are alterable. Today’s PLCs include those that use nonvolatile memory, those that use volatile memory with battery backup, as well as those that offer both.

There are two major concerns regarding the type of memory where the application program is stored. Since this memory is responsible for retaining the control program that will run each day, volatility should be the prime concern. Without the application program, production may be delayed or forfeited, and the outcome is usually unpleasant. A second concern should be the ease with which the program stored in memory can be altered. Ease in altering the application memory is important, since this memory is ultimately involved in any interaction between the user and the controller. This interac- tion begins with program entry and continues with program changes made during program generation and system start-up, along with on-line changes, such as changing timer or counter preset values.

The following discussion describes six types of memory and how their characteristics affect the manner in which programmed instructions are retained or altered within a programmable controller.

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